In the field of transportation equipment such as automobiles, an attempt is extensively made to reduce the mass by using high-strength materials. For example, in automobiles, use of high-strength steel sheets has been steadily increased with an intention to improve collision safety and enhance functionality without increasing the car body mass, and also improve fuel efficiency to reduce emissions of carbon dioxide.
In this movement for expansion of use of high-strength steel sheets, the biggest problem is manifestation of a phenomenon called “degradation of shape fixability”, which is more likely to occur as the strength of the steel sheet is increased. The phenomenon is more likely to occur as the spring back amount after forming increases with strength enhancement, and the phenomenon causes such an additional problem specific to high-strength steel sheets that it is not easy to obtain a desired shape.
For solving the problem, it is necessary in a usual method for forming a high-strength steel sheet additionally to carry out an unnecessary processing step (e.g. restriking) for a low-strength material free from the problem of degradation of shape fixability, or to change the product shape.
As one method for solving such situations, a hot-forming method called a hot stamping method has received attention. The hot stamping method is a method in which a steel sheet (processed material) is heated to a predetermined temperature (generally the temperature that serves as an austenite phase), and stamped by a die having a temperature (e.g. room temperature) lower than the temperature of the processed material with the strength of the processed material decreased for facilitating forming, whereby a desired shape can be easily provided, and also a rapid cooling heat treatment (quenching) using a difference in temperature between the processed material and the pressing is performed to increase the strength of a product after forming.
In recent years, the hot stamping method has been recognized for its usefulness, and a wide range of steel materials have been considered to be applied. Examples thereof include steel materials that are used under a severe corrosive environment, like automobile undercarriage components, and steel materials provided with perforated portions for the purpose of joining other components. Thus, steel materials obtained by the hot stamping method have been required to have not only strength but also hydrogen embrittlement resistance.
This is because while it is generally known that hydrogen embrittlement resistance is reduced with strength enhancement of steel materials, a steel material obtained by the hot stamping method generally has high strength, and therefore in application of the hot stamping method to the steel material, the steel material is exposed to a corrosive environment to accelerate ingress of hydrogen into the steel, and massive residual stress occurs as processing such as punching is performed, thus raising the possibility that hydrogen embrittlement occurs.
From such a viewpoint, a technique intended to secure hydrogen embrittlement resistance has also been proposed for steel materials whose strength is enhanced by the hot stamping method. For example, Patent Literature 1 discloses a technique concerning a steel sheet having resistance to delayed rupture (the same meaning as hydrogen embrittlement resistance) by including at a predetermined density one or more of oxides, sulfides, composite crystallized products and composite precipitated products of Mg having an average particle size in a predetermine range. Patent Literature 2 discloses a technique in which the punching characteristic is improved by performing punching (perforation) in a high-temperature state (hot) after heating for hot stamping and before pressing, so that delayed rupture resistance is improved.